Chemical nickel plating processes and baths therefor



United States Patent O CHEMICAL NICKEL PLATING PROCESSES AND RATESTHEREFOR Gregoire Gutzeit, Highland, Ind., PaulTalmey, Earrington, Ill.,and Warren G. Lee, East Chicago, 1nd,, assignors to GeneralAmerican'Transportation Corporation, Chicago, 111., a corporation of NewYork Application December 31, 1954, Serial No. 479,088

7 Claims. (Cl. 117-130) The present invention relates to improvedprocesses of chemical nickel plating of catalytic materials employingbaths of the nickel cation-hypophosphite anion type and to improvedbaths therefor, and more particularly to such processes and bathsinvolving a continuous system of the character of that disclosed in U.S. Patent No. 2,658,839, granted on November 10, 1953, to Paul Talmeyand William J. Crehan. This application is a continuation-in-part of thecopending application of Gregoire Gutzeit, Paul Talmey and Warren G.Lee, Serial No. 478,492, filed December 29, 1954; and the lastmentionedapplication is, in turn, a continuation-in-part of the abandonedapplication of Gregoire Gutzeit, Paul Talmey and Warren G. Lee, SerialNo. 376,968, filed August 27, 1953.

The chemical nickel plating of a catalytic material employing an aqueousbath of the nickel cation-hypophosphite anion type is based upon thecatalytic reduction of nickel cations to metallic nickel and thecorresponding oxidation of hypophosphite anions to phosphite anions withthe evolution of hydrogen gas at the catalytic surface. The reactionstake place when the body of catalytic material is immersed in theplating bath, and the exterior surface of the body of catalytic materialis coated with nickel. The following elements are catalytic for theoxidation of hypophosphite anions and thus may be directly nickelplated: iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium,iridium and platinum. The following elements are examples of materialswhich may by nickel plated by virtue of the initialdisplaceme'ntdeposition of nickel thereon either directly or through agalvanic effect: copper, silver, gold, beryllium, germanium, aluminum,carbon, vanadium, molybdenum, tungsten, chromium, selenium, titanium anduranium. The following elements are examples of non-catalytic materialswhich ordinarily may not be nickel plated: bismuth, cadmium, tin, leadand zinc. The activity of the catalytic materials varies considerablyand the following elements are particularly good catalysts in thechemical nickel plating bath: iron, cobalt, nickel and palladium. Thechemical nickel plating process is autocatalytic since both theoriginalsurface of the body being plated and the nickel metal that is depositedon the surface thereof are catalytic; and the reduction of the nickelcations to metallic nickel in the plating bath proceeds until all of thenickel cations have been reduced tometallic-nickel, in the presence ofan excess of hypophosphite anions, or until all of the hypophosphiteanions have been oxidized to phosphite ions, in the presence of anexcess of nickel cations.

In a batch plating process, the reactions are sloweddown rather rapidlyas time proceeds because the anions, as contrasted with the cations, ofthe nickel salt that is dissolved in the plating bath combine with thehydrogen cations to form an acid, which, in'turn, lowers the pH of thebath, and the reducing power of the hypophosphite anions is decreased asthe pH value of the bath'decreases.

2,822,293 Patented Feb. 4, 1958 Moreover, there is a tendency for theearly formation in objectionable in that it causes the nickel deposit tobe coarse, rough and frequently'porous. Any fine solid particlessuspended in the plating bath, or adhering to the walls of the platingvessel, at the plating temperature, initiate the formation of the blackprecipitate by acting as nuclei.

In a continuous plating process the reactions are maintainedsubstantially at their initial rates by the regeneration of the platingbath, i. e., by the adding thereto of soluble nickel-containing andhypophosphite-containing reagents, as well as an alkali for pH control;however, the problem of preventing the formation of black precipitate inthe plating bath and the consequent decomposition=thereof is the sameas-that previously mentioned. Moreover, another practical difficulty isencountered in the continuous plating process that is not encountered inthe batch plating process in that there is a considerable build-up ofthe 'by-product phosphite therein as time proceeds and as aconsequenceof the cycling of the bath. More particularly, while nickelhypophosphite is readily soluble in anaqueous solution, nickel phosphiteis much less'soluble in an aqueous solution; whereby there is atendency, as the phosphite concentration of the plating bathbuilds-up,for nickel phosphite to be precipitated therein, and thereby provide thesolid particles that serve as nuclei for the formation of the blackprecipitate therein, previously mentioned. In passing, it is noted thatthe initiation of the precipitation of nickel phosphite in the platingbath is indicated byturbidity thereof, visible in a Tyndall beam.

'In carrying out the chemical nickel plating process on a commercialscale, the continuouszsystem-Idisclosed in the Talmey and Crehan patentmay be employed; which system involves periodic or continuousregeneration of the plating bath by the addition thereto of appropriateingredients for the purpose of-maintaining substantially constant thecomposition of the bath, as previously noted. More specifically in thissystem, there are provided a plating chamber and a reservoir; preferablyone portion of the plating solution is stored at a relatively lowtemperature well below the boiling point: thereof in the reservoir; andpreferably another portion of the plating solution is held as a bath ata relatively high temperature slightly below the boiling point thereofin the plating chamber. The solution is continuously circulated at a lowrate from the reservoir to the .platingschamber and then back to thereservoir, the solution being heated substantially to the relativelyhigh temperature after withdrawal thereof from thereservoir and beforeintroduction thereof into thev plating chamber, and the solution beingcooled substantially to the relatively low temperature after withdrawalthereof from the platingchamher and before return thereof to thereservoir?" Thebody that is to be nickel plated is immersed in the bathin the plating chamber andis subsequently withdrawn from the bath in theplating chamber after a time interval corresponding to the thickness ofthe nickelplating thereon that is desired; and duringsuch-time"interval' soluble reagents are added to the solution inthereservoir to maintain in the bath in the plating chamber during suchtime interval substantially thepredetermined composition of the bathpreviously mentioned, so as to compensate for the ingredients of the'bath thatare exhausted during-the time interval in the platingchamberlThis regeneration of the solution in the "reservoir consists essentiallyof adding thereto appropriate-amounts of soluble nickel-containing andhypophosphite-containing reagents, as well as an alkali for pH control,as pre viously noted.

In a plating bath of the nickelcation-hypophosphite anion type, thethreshold of precipitation of the insoluble nickel phosphite begins whenthe (HPO concentration attains a value above the solubility of itssimple nickel salt or its double nickel-alkali salt; i. e., above 0.03to 0.07 m. p. 1.; and in order to obviate this defect, there aredisclosed in the previously mentioned abandoned application of GregoireGutzeit, Paul Talmey and Warren G. Lee, Serial No. 376,968, filed August27, 1953, modified plating baths of the nickel cation-hypophosphiteanion type containing both complexing agents and exalting additives. Inthese plating baths, the complexing agents serve to tie-up the nickelions, thereby preventing precipitation of nickel phosphite until a highconcentration of phosphite ions (about 1.0 m. p. l.) is reached in theplating bath in the continuous plating system; and the exaltingadditives serve to increase the normally low plating rates of thesebaths containing the complexed nickel ions. Among the complexing agentsdisclosed, those forming water-soluble chelates are most efiicient; and,within that group, the hydroxycarboxylic acids have several practicaladvantages, such as: ready availability, low price and high bufferingcapacity.

More particularly, it is apparent that if the nickel ions are verystrongly tied-up (i. e., if the chelate is very stable), they areactually removed from the plating process, and no longer available fordeposition; whereas, if the complex bond energy is at a lower level, anequilibrium is reached between the dissociation rate of the nickelcomplex ion and the deposition rate of metallic nickel. The stability ofnickel chelates with various hydroxycarboxylic acid additives is notonly a function of the number of hydroxyl and carboxyl groups in theacid molecule, but also of molecular structure and steric factors, asmay be better appreciated from a consideration of the structure of themore common. of these acids:

Glycollic acid (hydroxyacetic acid) H2CC OH OH Malic acid(monhydroxysuccinic acid) 0 H Hi l-o 0 on E2 (3-0 0 o H Lactic acid(alpha-hydroxypropionic acid) CH; HO-(BH coon Tartaric acid(dihydroxysuccinic acid) OH HiJ-O 0 0H HCC 0 OH Citric acid HaC- c 0 o H110- o-o 0 on H2 0- c 0 o E It is obvious that tartaric acid having twohydroxyl and two carboxyl groups will give the most stable complex; andit is also normal that glycollic and lactic acid complexes will show theleast stability, both being monohydroxy-monocarboxylic acids. On theother hand, the lactic chelate of nickel is less stable than theglycollic complex; and this is due to a structural factor, i; e., the

presence of an additional methyl group (CH Moreover, chelate stabilityis also determined by the number of carboxyl groups in the molecule sothat the nickelmalic acid complex (a monohydroxy-dicarboxylic compound)is more stable than the corresponding chelates of both glycollic andlactic acids (monohydroxy-monocarboxylic acids), while the citric acidcomplex is the most stable of all.

Generally, if hydroxycarboxylic acids instead of a nonchelating butter)are added to a chemical plating bath of the nickel cation-hypophosphiteanion type in continuous operation, the resulting plating rate will bean inverse function of chelate stability; however, on the other hand,the more stable the nickel complex, the higher a phosphite ionconcentration can be built-up before precipitation of nickel phosphiteoccurs.

For the above reasons, the plating baths disclosed in the Gutzeit,Talmey and Lee application, Serial No. 478,492 comprise relativelystable complexing agents in combination with powerful exalting additivesdicarboxylic acids, aminocarhoxylic acids and certain monocarboxylicacids) to increase the normally low plating rates of these baths.

The present invention is predicated upon the discovery that in a platingbath of the nickel cation-hypophosphite anion type described, while thenickel chelating function of lactic acid is directly proportional to theconcentration thereof in the bath, the effect thereof upon the platingrate of the bath is not inversely proportional to the concentrationthereof as is the general case with the other hydroxycarboxylic acids);rather within a given range of concentration, lactic acid is a definiteexaltant in the bath, substantially increasing the plating rate thereof.The mechanism of this exalting effect is not fully understood, but theeffect is very pronounced; and it is most unusual and entirelyunexpected that lactic acid unlike the other common hydroxy-earboxylicacids) should possess this particular characteristic.

The general composition of a plating bath in accordance with the'presentinvention essentially comprises an aqueous solution of a nickel salt, ahypophosphite, and lactic acid or a salt thereof; wherein the absoluteconcentration of hypophosphite anions in the bath is in the range 0.15to 1.20 m. p. l., the ratio between nickel cations and hypophosphiteanions in the bath expressed in molar concentrations is within the range0.25 to 1.60, and the ratio between nickel cations and lactic ions inthe bath expressed in molar concentrations is within the range 0.15 to0.35. The pH of the bath is normally in the range 4.0 to 5.6; and thebath is employed in the plating chamber of the continuous plating systemat a temperature above C., ordinarily slightly below the boiling pointthereof and at about 97 to 99 C. The bath has a nickel plating rate ofat least 1 mil/hour (0.00l"/hour), or expressed in c. g. s. units of atleast 3.5X10- gm./cm. /min.; and no precipitation of nickel phosphitetakes place therein even at a phosphite ion concentration in some casesvery close to 1.0 m. p. 1. Further, the plating appearance on bothmetals and nonmetals is excellent (bright, smooth and non-porous); and

adhesion of the nickel plating on both metallic and nonmetallic bodiesis excellent (no flaking of the nickel coat ing in bending, abrading andshock tests).

In accordance with the process of the present invention, the platingbath of the composition specified is preferably employed in thecontinuous plating system of the character previously described, wherebythe lactic acid additive is present therein in the optimum rangespecified so. that it serves both the complexing or chelating functionwith respect to the nickel ions and also the function of increasing theotherwise relatively low plating rate of the bath. This complexing ofthe nickel cations in the plating bath prevents the formation ofprecipitated phosphite therein, thereby rendering the bath ofexceedingly long life in spite of the build-up of phosphite ions thereinto a concentration even in excess of one molar. This complex'of nickelin the plating bath is water-soluble and of medium stability resultingin a bond strong enough to prevent the 'nickel cations from forr iiing-insoluble nickel compounds, but having a stability constant low enoughto release the nickel cations required for the nickel plating operationto effect a plating rate of the bath of at least 35x10" gm./cm. /min.,as previously explained.

In view of the foregoing, it is the primary object of the presentinvention to provide an improved nickel plating process of the characterdescribed in which the reactions involved are carried out moreefficiently and under more stable conditions (clarity of solution) thanheretofore, thereby rendering the process more desirable from acommercial standpoint.

Another object of the invention is to provide an improved aqueouschemical nickel plating bath that may be employed with advantage in thepractice of the improved process.

Another object of the invention is to provide an improved nickel platingprocess of the character described,

that employs a plating bath of the nickel cation-hypo phosphite aniontype of the character specified, wherein the plating rate thereof to atleast 3.5 lgm./cm.

:min.

A further object of the invention is to provide an improved nickelplating process of the continuous type involving an improved platingbath of the nickel cationhypophosphite anion type, so that the usefullife of the bath is greatly extended in that it remains clear,notwithstanding the presence therein of a phosphite anion concentrationapproaching one molar.

A still further object of the invention is to provide an improved nickelplating bath of the character described that involves a novel range oflactic ion addition.

These and other objects and advantages of the invention pertain to theparticular arrangement of the steps of the plating process and of thecomposition of the plating bath, as will be understood from theforegoing and following description, taken in connection with theaccompanying drawing, in which:

Figure l is a series of curves illustrating the relationship between theplating rates of a number of plating baths of the nickelcation-hypophosphite anion type and the concentrations of several commonhydroxy-carboxylic acids contained therein; and

Fig. 2 is a curve illustrating the relationship between phosphitetolerance of a plating bath of the type mentioned and the concentrationof lactic acid contained therein.

In accordance with the process of the present invention, the article tobe nickel plated and normally having a catalytic surface is properlyprepared by mechanical cleaning, degreasing and light picklingsubstantially in accordance with standard practices in electroplatingprocesses. For example, in the nickel platingof a steel article, it iscustomary to clean the rust and mill scale from the article, to degreasethe article and then lightly to pickle the article in a suitable acid,such as hydrochloric acid. The article is then immersed in a suitablevolume of the plating bath containing the proper proportions of nickelcations, hypophosphite anions and lactic ions, the pH of the bath havingbeen, if necessary, adjusted to an optimum value by the addition of anappropriate acid or base, and the bath having been heated to atemperature just below its boiling point, such as 99 C. at atmosphericpressure. Almost immediately, hydrogen bubbles are formed on thecatalytic surface of the steel article and escape in a steady streamfrom the plating bath, while the surface of the steel article is slowlycoated with metallic nickel (containing some phosphorus). The steelarticle is subsequently removed from the bath after an appropriate timeinterval corresponding to the required thickness of the nickel coatingdeposited thereon that N 6 is desired; and'ulti'matelythe steel "articleis rins'ed'otf with water, so that it is ready for use.

In the plating bath, the nickel cations may be derived from nickelchloride, nickel sulfate, etc., or various combinations thereof; thehypophosphite anions maybe derived from sodium hypophosphite, potasiumhypophosphite, etc., or various combinations thereof; and the lacticions may be derived from lactic acid, or various lactates, or variouscombinations thereof. The desired pH of the bath is established by theeventual introduction thereinto of hydrochloricacid or an alkali, suchas sodium hydroxide, sodium carbonate or sodium bicarbonate.

The terms cation, anion and ion as employed herein, except wherespecifically noted, include the total quantity of the correspondingelements that are present in the plating bath, i. e., both undissociatedand dissociated material. In other words, 100% dissociation is assumedwnen the terms noted are used in connection with molar ratios andconcentrations in the plating bath.

In order to demonstrate the remarkable advantages of the plating bath ofthe present invention, first and second series of reference platingtests (10 minute rate tests) were conducted employing standard steelsamples that had been given a standard pretreatment. More particularly,steel samples (Dayton Rodgers) of 20 cm. total area were vapordegreased, cleaned by an alkaline soak and lightly pickled in 1:1hydrochloric acid. The steel samples thus prepared were then plated at981- 1 C. in 50 cc. of different plating baths each containingnickelcations (as nickel sulfate) 0.08 m. p. l. hypophospnite anions (assodium hypophosphite) 0.224 m. p. l. and the indicated amounts oforganic additives (acetate ions or hydroxycarboxylic ions), the latterbeing equivalent with respect-to their hy droxylgroups, and the pH beingadjusted with NaOH'to about 4.7.

The results of the first series of reference plating tests employingsodium acetate are shown in the table below:

TABLE I 0.16 0.30 Weight gain (gm.)..... 0.0993 0.1018 Plating rate, R10 gm cm 4. 97 5.09 Initi pH 4. 67 4. 70 Final pI-I 4.00 4. 31

Bath appearance Plating appearance 1 Clear 2 Smooth and dull.

The results of the second series of reference plating tests employingvarious hydroxycarboxylic acids, and one without an organic additive forreference purposes are shown in the table below:

TABLE II Hydroxycarboxylic Anlons Gly- Malic Tar- Citric None collietaric Hydroxyl groups Concentration, in. p. 1 Weight gain (gm) Platingrate, RX10 gm./

cmJ/rnin Initial pH. Final pH Bath appearanc Plating appearanc zation(steam entrainment);

Moreover, in these 10- minute rate tests, the plating rates aremuchhigher than in comparable 1-hour rate tests, but they are valid fordynamic (or cyclic operation), in which the bath is continu- Anexamination of Table IV reveals that an increase in the concentration ofchelating agent (hydroxycarboxylic acid decreases the plating ratealmost as a linear function of reagent concentration and complexstability;

ously circulated and regenerated. While the plating 5 which relationshipis graphically illustrated by the curves baths of Table I have highplating rates, these baths are 11 and 12 in Fig. 1, respectivelycorresponding to malic totally unsuitable for continuous operation inthat acid and citric acid. they lack stability, black precipitate beingformed therein In this connection, it is noted that for completestoiwhen the phosphite concentration reaches a value as low chiomelfihcomplexihg, each 111018 of nickel requires at as 0.03 m. p. l. in thebaths. a least two carboxyl groups and at least one hydroxyl group.

R f i now t th fi t f l i b th f T bl Thus two moles of glycollic acidor lactic acid are neces- H (the baths respectively containingglycollic, malic, tarsary P 31016 of nickel, While one mole of tartaricacid taric and citric anions), it will be appreciated that the 01' malicacid Will 61181316 one mole Of nickel. on the plating rates thereof arevery low, particularly with malic, other hand, two moles of citric acidwill chmplex thl'ee tartaric and citric anions, the citrate being thebest buffer 15 moles of nickel- Accordingly, in these Plating baths of(no pH change in IO-rninutes). The low plating rate of the nickelcahoh-hypophhsphite ahhm yp the amounts h fifth l ti b h of T bl 11 h bh containing of hydroxycarboxylic acids sufficient to achieve completeno organic additive) is, of course, due to the rapid rise stoichiomeh'iccomplexihg of the nickel are Set forth in in hydrogen ion concentration(reduction in pH), there table below! being no buffering action so thatthe reducing efficiency of TABLE V the hypophosphite is rapidlydecreased, plating stopping altogether at a pH below about 3.0.Hydroxycarboxylic Anions Gly- Lactic Malic Tar- Citric Then a thirdseries of reference plating tests were conmum mm ducted under conditionsidentical to the second series of reference plating tests describedabove, except that in the gfifif gg gg gfigiiigg l 1 2 2 3 plating bathsthe amounts of the hydroxycarboxylic acid P- N1 0-053 additives wereselected so that they were equivalent with respect to the carboxylgroups. The results of this third As disclosed in thepreviously-mentioned application series of reference plating tests areshown in the table of GlltZeit, Talmey and Serial P g below: bathscontaining hydroxycarboxylic acids in the concen- TABLE III trationsindicated in Table V above give continuous plating rates (without theaddition of exaltants) of only about Hydroxycarboxyllc Anions GlycollieMalic Tartaric Citric L80X10-4' gm-/cm-2/min- Now it has been discoveredthat by increasing the procgrboxylgl 'gups 1 2 2 3 portion of lacticacid in these plating baths of the nickel iigiitiiifiiaiiffli:::: acid0. 321% 0.35%? with catim'hypwhmfhhe type Precisely a Platlng rate, x 4gmJgmfl/ ratio between nickel cations and lactic acid anions of 2-3; i igg about 1:4), there results both a relatively high phosphite 4: 1 1 1tolerance (almost one molar) and a plating rate in excess (9 w (9 of'3.5 10- gm./cm. /rnin. (corresponding to 1 mil/ hour). and bright g Inanother plating test, conducted under conditions From a companson ofTables H and In, W111 identical to the second and third series ofreference platobserved that the results of the second *andthrrd serles45 ing tests described above, a plating bath containing lactic ofreference plating tests were substantially identical. acid in theconcentration noted immediately above was Then a fourth seriesofreference plating t s s were employed; and the results of this platingtest, together conducted under conditionsudentical to the second andwith a recapitulation of the results of the tests of the th1rd series ofreference plating tests described above, exsecond and third Series ofreference plating tests are Set cept that 1n the plating baths only thechelating agents, forth in the table below.

o o mahc acid and citric acid, were involved and the con- TABLE VIHydroxycarboxylic Glycollie Mallc Tartaric Citric Lactic AnionsConcentration, in. p. 1..-. 0.30 0. 15 0. 30 0. 15 0.10 0.30 0.30 Weightgain (gm.) 0. 0502 0.0470 0. 0357 0.0305 0. 0309 0. 0032 0 0700 Platingrate, RX104 gnn/ anal/min 2.51 2.35 1.79 1. 5a 1. .55 0.10 3. 5a 4.68 4.71 4.08 4. 72 4. 68 4.07 4.73 4. 23 4.35 4. 4.28 4.42 4. 67 4.02 hDpearanc 0) Plating appearance 1 Clear.

2 Smooth and bright. V

centration thereof was varied in the different baths. The results ofthis fourth series of reference plating tests are shown in the tablebelow:

TABLE IV Accordingly, from Table VI, it will be observed that lacticacid (a weak chelating agent) has the advantages inherent to allhydroxycarboxylic anions as far as preventing phosphite precipitation isconcerned, and in addition thereto the function of an exaltant. Further,the alkali salts of lactic acid (the lactates) are good buffers in theproper pH range.

A further series of plating tests were conducted under conditionsidentical to the second and third series of reference plating testsdescribed above, except that in the plating baths variableconcentrations of lactic acid anions stat were employed. The results ofthese ing rate is exalted by an increased concentration of lactic acidin the platingbath.

TABLE VII As is usual with chemical nickelplating baths of .the

buffered nickel cation-hypophosphite type, the plating rate 5 is afunction of pH; and in order to demonstrate this igg iiggf fg i M8 160.24 030 M0 M0 characteristic, stillanother series of plating tests wereconlriVleaigillllt .2 0.0503 0.0759 0.0701 0. 0706 0.0092 0.0002 ductedem loying steel samples (Dayton Rodgers Shimfi fi 282 380 396 M3 M6 01stock) of 20 cm. total area that had been vapor degreased,

rgtm- 2 5g ii-g; electrocleaned and lightly pickled in dilutehydrochloric nan i sane... o o o 0 (0' -.1 nletma s en r ens x gl aggagp e ggt g p utesat 98 C. ;1 C. n 50 cc. of the plating bath havmg N t,i 6( 8% g 38 fi the following fundamental compositlonz e Nickel ion 0.08in. 1. (NiSO '6H O).

Clean Hypop hosp to 'on 0.225 m. p. l..(NaH PO 1 Smooth and bright.Lactic ion u 0.40 m. p. l. (lactic acid).

h e l of h se plating tests are represented p In these baths,. therelatively high concentration of lacal y yt t 13 in 1, employing thedata of tie. ions is necessary for stability (prevention of formationTable VII a ve;.w y the relationship between the of. black precipitate)at elevated .pH values employed in plating rate andthe concentration oflactic acid is unique h l i t a d i th plating te t the pH of thein.that there is a defini m m m pl ing r wi h baths were adjustedwithNaOH. The results of these Ni++ /lactic ion ratios at about 1:3. platingtests are shown in the table below:

ABL VI Duration of test, min-.- 10 6 0 InitialpH 4.5 5.0 5.5 5.0 6 5 4.55.0 5.5 0.0 0.5 FmalpH 4.0 4.0 4.1 4.1 4.1 3.8 5.9 3.95 4.0 4.0 We1ghtgaln(gm.) 0.0699 0 0702 0. 0826- 0. 0832 0.0851 0.1862 0.1920 0 19280.1939 0 1844 Platlng rate, RX104 gnm/cmfi/mim--. 3.5 3.81 4.13 4.124.26 Plating pp 1 Bright and smooth.

Furthermore, the phosphite tolerances in m. p. l. of From Table VIII, itwill be observed that a maximum these plating baths of Table VII areillustrated graphically platingrate is obtained around a pH of 6.0;however, in by the curve 21 in Fig. 2; whereby it is apparent that the acontinuous plating operation; phosphite precipitation phosphitetolerance thereof increases disproportionally takes place at a lowerconcentration of I-IP0 a't1 with an increase in the lactic acidconcentration therein. orhydrogen ion concentrations; andmoreoveradhesion to Also in t s Connection, it is Pointed out t a thebase metals is very supreior at pH values below 5.0. stan ial yidentical Plating bath containing 030 P- For these particular, reasons,the preferred pHis between of acetic anions has a phosphite tolerance ofonly 0.05 4.4 and 5.6 despite a lower plating rate. .Howevel, the m. p.1.; whereas a substantially identical plating bath conbath is usefulover, the relatively wide approximate pH taining as little as 0.10 m. p.l. of citric acid has a phos- 5 range 4.0 to 5.6. phite tolerance inexcess of 2.0 m. p. 1. In these plating baths, it is, of course,permissible to Accordingly, it Will be appreciated from a study of thevary th e nickel cation and hypophosphite anion concendata of Table VII,together With the C e 13 f g- 1 trations within limits, provided theNi+'+/Iactic "anion and the Curve 21 0f that the maXimum P t g ratioremains within the indicated optimum range of 'ab out rate is obtainedwhen all of the nickel has been complexed 5 115:1); h b lut n t tio of hpo' h 'hit and enough additional lactic anion is present to form a i ith b th, expressed i mol /liter, may be within mone-heteropolya i w h hhyp ph p radical the range 0.15 to 1.20; and the .ratioibetweennickelions 0115 additional mole), while the phosphite tolerance iS andhypophos phite ions in the bathpe xpressed 'in molar strictly a functionof available excess of chelating agent. concentrations, may be withinthe rangeofii's to 1.6 0. However, in order to obtain optimum Ov r lresults in a In order further to illustrate the practical range of thecontinuous plating system, it is not possible to ke a Ni++/lactic anionratio, companion plating tests were cont e of the maximum plating rate(which Occurs when ducted under conditions identical to th osepreviously dethe Ni++/1acti i n rati s about t a compromised scribed in10-minute ratet'estseniplo in two baths as folvalue for the Nit /lacticion ratio has to be chosen so lo s; that the plating rate remains above3.5 l0 Bath I gm./cm. /min., while the phosphite tolerance is as high aspossible. This optimum region, based upon both of the g i T *'T"-"*considerations mentioned immediately above, is around a 29 5 1 e 1911 6p Ni++/lactic ion ratio of about 145:1 or expressed nu- 6 mericallyabout 0.15to 0.25. However, the bath may con- "TT-" a tain lactic anionsin the approximate range 0.25 to 0.60 1 acne Ion m lpfil. h f h b t 1 dBath ll e mec anism o t e corn Ina ion comp exmg an eX- alting effectsof lactic acid in the plating bath is not alto- Nickel i? 0'0675 getherunderstood as it is entirely different from that of any Hyp9phOp lie0225 other hydroxycarboxylic acid with reference to the plating Lacncacid 0400 rate. In other words, instead of depressing the platingH;;"".I'"* (NaOH)' rate directly in proportion to concentration, as isthe nor- N1 name Ion mal characteristic of other commonhydroxycarboxylic In these. companion plating tests, the respectiveplating acids, there is a definite optimum range in which the plat- 75rates were 4.21 forBath I and 3.84 for Bath II (expressed 11 g 1 in Rgm./cm. /rnin.); and the phosphite tolerance (using C. P. Na HPO jH O)was better than 0.9 m. p. l. of (HPO for Bath 1. T a

In a continuous plating system, a plating test was per; formed involvinga number of steel samples prepared in the manner previously explainedand utilizing a similar plating bath having the following initialcomposition:

Bath III Nickel ion 0.07 m. p. l. (as nickel sulfate). Hypophosphite ion0.225 m. p. l. (as sodium hypophosphite). Lactic ion 0.300 m. p. l.(aslactic acid).

The initial pH of Bath III was adiusted to 4.55 with caustic soda; andat the conclusion of the dynamic run, it was determined that the averageplating rate upon the samples was 0.9 mil/hour, that the hypophosphiteutilization was the usual 33.33%, and that the phosphite tolerance was0.953 m. p. l. The plating was analyzed as 90.5% nickel and 9.2%phosphorus; the hardness of the plating was from.500 to 600 Vickcrsnumber, with 70% of the samples being 537 V.

Then in the continuous plating system, a similar plating test wasperformed involving a number of cycles of circulation of the body ofplating solution and including periodic regeneration of the plating bathin the reservoir exteriorly of the plating chamber, as disclosed in thepreviously mentioned Talmey and Crehan patent. In this plating test, anumber of cold rolled steel samples (1%" x 6" x 14 ga.) were employedthat had been subjected to a pretreatment including vapor degreasing,alkaline soaking and pickling in 2:1 hydrochloric acid. In this platingtest, the initial composition of the plating bath was as follows:

Bath IV V Nickel ion; 0.08 m. p. l. (as nickel sulfate). Hypophosphiteion 0.23 m. p. l. (as sodium hypophosphite). Lactic ion 0.30 m. p. 1.(as lactic acid).

The initial pH of plating Bath IV, was adjusted to 4.50 with causticsoda; and the specific results of this continuous plating test are setforth in the table below:

above, it is desirable to add a separate and'independent exaltant,because in continuous operation the complexing action of the lactic acidpredominates; and while exaltants of the character disclosed in U. S.Patent No. 2,658,842, granted on November 10, 1953, to Gregoire Gutzeitand Ernest J. Ramirez (simple short chain saturated aliphaticdicarboxylic acids and salts thereof) are highly satisfactory, it iseconomically advantageous to employ as an exaltant a simple short chainsaturated monocarboxylic acid (acetic, propionic, buytric and valeric),particularly in view of the solubility of their calcium salts, as willbe shown below.

More specifically, in the operation of the continuous plating system,after a relatively long time interval of production plating, thephosphite concentration buildsup to a point where a slight excess of(HPO despite the presence of a complexing agent, will result in nickelphosphite precipitation; in other words, a threshold is reached wherethe solubility of nickel phosphite, even in the presence of a nickelchelating agent, is exceeded. At this time, it becomes necessary toremove, by some method, the excess phosphite ion, as well as the excesssodium and sulfate ions that have accumulated as a result ofregeneration. A simple and economical method of achieving this objectiveis disclosed in the copending application of Paul Talmey, GregoireGutzeit and Donald Metheny, Serial No. 479,040, filed December 31, 1954,now abandoned, and involving the addition to the spent plating bath of aslight excess of calcium hydroxide, resulting in the precipitation ofnickel phosphite, calcium phosphite and calcium sulfate; whereby, forpractical reasons, it is highly desirable to employ in these platingbaths a simple short chain saturated aliphatic monocarboxylic acid as anexaltant, which is not removed in the above described process, becauseits calcium salt is soluble. Also it is noted that the calcium salts oflactic acid are soluble, while most common hydroxycarboxylic acids forminsoluble calcium compounds (malic, citric, tartaric, etc.).

The process and the bath comprising both lactic acid and a separate andindependent exalting additive selected from the group consisting ofpropionic acid, butyric acid and valeric acid is disclosed and claimedin the copend- TABLE IX Cycle No 1 2 3 4 5 6 7 8 9 10 Total weight gain(gm) 5.0 5. 0 4. 7 4.0 3. 6 2. 6 2.6 2.80 2. 95 3.00 Plating rate, Rxlfl(gm.lcm.lmln.) 3. 08 3. l3 2. 95 2. 71 2. 44 3. 90 3. 90 3. 45 3. 70 3.76 Plating rate (miL/hr.) 0. 88 0. 89 0. 84 0. 77 0.70 1.11 1.11 0.09 1. 06 1. 07 Soln. flow rate (co/min. 105 v 105 105 105 105 105 105105 105 105 Plating time (min.) 56 55 51 51 46 46 56 55 55 Initial p 4.50 4. 51 4. 50 4:50 4. 51 4. 50 4. 50 4. 50 4. 50 4. 50 Ni added (gm)(as NlSO 611 0) 4.65 4.65 4.37 3. 72 3.24 2. 40 2. 40 2. 2. 79 2. 79NaHzPO: added (gm.). 24 24 23. 6 20.0 17. 3 15.6 15. 6 14. 0 14.5 14. 5NaOH added (gm.) 6. 3 6.3 5. 9 5.0 4.3 3. 9 3. 9 3.1 3. 6 3. 6 Additive(p. p. m.) Pb++.. 1.0 0. 5 0.5 0 0.5 0 0 0.5 0 0 Ni turnover (mol/liter)0. 0132 0. 0264 0. 0388 0. 0494 0. 0586 0.0659 0. 0723 0. 0796 0. 08750. 0981. Plating appearance 0) (0 1 All samples semi-brighfi and smooth.

In cycles 1 to 5, inclusive, of this plating test, two of the panelsspecified having a total area of 290 cm. were plated; whereas in cycles6 to 10, inclusive, of this plating test, only one panel specifiedhaving a total area of 145 cm. was plated. Furthermore, at the beginningof cycle 8 of this plating test, an extra 4.0 gm. of NiSO .6H O wasadded, as an analysis showed that the nickel content of the plating bathwas low. Also, the plating rates obtained in cycles 6 and 7 of thisplating test were clearly indicative of the circumstance that the totalcontent of Ni++ was running lower than that desired.

In plating operations involving plating baths of the nickelcation-hypophosphite anion type containing lactic ion as the complexingand exalting agent, as disclosed ing application of Gregoire Gutzeit,Paul Talmey and 'Warren G. Lee, Serial No. 569,815, filed March 6, 1956.

In this connection, it is pointed out that as the phosphiteconcentration builds-up in a plating bath of this type, as-the platingbath is used in the continuous plating system, the plating rate of thebath declines. Accordingly,

it will be appreciated thatthe level of concentration of the phosphitein a plating bath of this type must be mainthe plating bath must bediscarded for further use, when about one-half to. three-fourth of thephosphite tolerance thereof is reached, thereby to insure that in theplating 13 operation there is no formation of black precipitate in theplating bath.

In view of the foregoing, it is apparent that there has been provided animproved process of chemical nickel plating, as well as improved platingbaths therefor, wherein the baths are of the nickel cation-hypophosphiteanion type and also containing as a combination complexing agent andexalting additive a predetermined range of lactic acid.

Further, these baths may advantageously contain as a separate andindependent exalting additive a simple short chain saturated aliphaticmonocarboxylic acid, preferably propionic acid, as disclosed in, thepreviously mentioned copending divisional application of Gutzeit, Talmeyand Lee. These plating baths are particularly welladapted for use in acontinuous plating system, as they exhibit a fast plating rate, have anexceedingly long life, are productive of entirely satisfactory platingquality, and maintain nickel phosphite in solution in concentrationsapproaching one molar.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:

l. The process of chemically plating with nickel a body essentiallycomprising an element selected from the group consisting of iron,cobalt, nickel, aluminum, copper, silver, gold, palladium and platinum,which comprises contacting said body with a bath consisting essentiallyof an aqueous solution of nickel ions, hypophosphite ions, and acomplexing agent selected from the group consisting of lactic acid andsalts thereof, wherein the absolute concentration of hypophosphite ionsin said bath expressed in mole/liter is within the range 0.15 to 1.20,the ratio between nickel ions and hypophosphite ions in said bathexpressed in molar concentrations is within the range 0.25 to 1.60, theabsolute concentration of lactic ions in said bath expressed inmole/liter is within the range 0.25 to 0.60, and the initial pH of saidbath is within the approximate range 4.0 to 5.6.

2. The process of chemically plating with nickel a body essentiallycomprising an element selected from the group consisting of iron,cobalt, nickel, aluminum, copper, silver, gold, palladium and platinum,which comprises contacting said body with a bath consisting essentiallyof an aqueous solution of a nickel salt, an alkaline hypophosphite, anda complexing agent selected from the group consisting of lactic acid andsalts thereof, wherein the absolute concentration of hypophosphite ionsin said bath expressed in mole/liter is within the range 0.15 to 1.20the ratio between nickel ions and hypophosphite ions in said bathexpressed in molar concentrations is within the range 0.25 to 1.60, theabsolute concentration of lactic ions in said bath expressed in mole/liter is within the range 0.25 to 0.60, and the initial pH of said bathis within the approximate range 4.0 to 5.6.

3. A bath for the chemical plating of a catalytic material with nickelconsisting essentially of an aqueous solution of a nickel salt, ahypophosphite, and a complexing agent selected from the group consistingof lactic acid and salts thereof, wherein the absolute concentration ofhypophosphite ions in said bath expressed in mole/liter is within therange 0.15 to 1.20, the ratio between nickel ions and hypophosphite ionsin said bath expressed in molar concentrations is within the range 0.25to 1.60, the absolute concentration of lactic ions in said bathexpressed in mole/liter is within the range 0.25 to 0.60, and the pH ofsaid bath is within the approximate range 4.0 to 5.6.

4. The process of chemically plating with nickel a body essentiallycomprising an element selected from the group consisting of iron,cobalt, nickel, aluminum, copper, silver, gold, palladium and platinum,which comprises contacting said body with a bath consisting essentiallyof an aqueous solution of nickel ions, hypophosphite ions, and acomplexing agent selected from the group consisting of lactic acid andsalts thereof, wherein the absolute concentration of hypophosphite ionsin said bath expressed in mole/liter is within the range 0.15 to 1.20,the ratio between nickel ions and hypophosphite ions in said bathexpressed in molar concentrations is within the range 0.25 to 1.60, theabsolute concenration of lactic ions in said bath expressed in mole/liter is within the range 0.25 to 0.45, and the initial pH of said bathis within the approximate range 4.4 to 5.6.

5. A bath for the chemical plating of a catalytic material with nickelconsisting essentially of an aqueous solution of a nickel salt, ahypophosphite, and a complexing agent selected from the group consistingof lactic acid and salts thereof, wherein the absolute concentration ofhypophosphite ions in said bath expressed in mole/liter is within therange 0.15 to 1.20, the ratio between nickel ions and hypophosphite ionsin said bath expressed in molar concentrations is within the range 0.25to 1.60, the absolute concentration of lactic ions in said bathexpressed in mole/liter is within the range 0.25 to 0.45, and the pH ofsaid bath is within the approximate range 4.4 to 5.6.

6. The process of chemically plating with nickel a body essentiallycomprising an element selected from the group consisting of iron,cobalt, nickel, aluminum, copper, silver, gold, palladium and platinum,which comprises contacting said body with a bath consisting essentiallyof an aqueous solution of nickel ions, hypophosphite ions and lacticions, wherein the absolute concentration of hypophosphite ions in saidbath expressed in mole/liter is within the range 0.15 to 1.20, the ratiobetween nickel ions and hypophosphite ions in said bath expressed inmolar concentrations is within the range 0.25 to 1.60, and the absoluteconcentration of lactic ions in said bath expressed in mole/liter iswithin the range 0.25 to 0.60.

7. A bath for the chemical plating of a catalytic material with nickelconsisting essentially of an aqueous solution of a nickel salt, ahypophosphite and a lactate, wherein the absolute concentration ofhypophosphite ions in said bath expressed in mole/liter is within therange 0.15 to 1.20, the ratio between nickel ions and hypophosphite ionsin said bath expressed in molar concentrations is within the range 0.25to 1.60, and the absolute concentration of lactic ions in said bathexpressed in mole/liter is within the range 0.25 to 0.60.

Brenner et a1. Dec. 5, 1950 Gutzeit et a1. Nov. 10, 1953

1. THE PROCESS OF CHEMICALLY PLATING WITH NICKEL A BODY ESSENTIALLYCOMPRISING AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF IRON,COBALT, NICKEL, ALUMINUM, COPPER, SILVER, GOLD, PALLADIUM AND PLATINUM,WHICH COMPRISES CONTACTING SAID BODY WITH A BATH CONSISTING ESSENTIALLYOF AN AQUEOUS SOLUTION OF NICKEL IONS, HYPOPHOSPHITE IONS, AND ACOMPLEXING AGENT SELECTED FROM THE GROUP CONSISTING OF LACTIC ACID ANDSALTS THEREOF, WHEREIN THE ABSOLUTE CONCENTRATION OF HYPOPHOSPHITE IONSIN SAID BATH EXPRESSED IN MOLE/LITER IS WITHIN THE RANGE 0.15 TO 1.20,THE RATIO BETWEEN NICKEL IONS AND HYPOPHOSPHITE IONS IN SAID BATHEXPRESSED IN MOLAR CONCENTRATIONS IS WITHIN THE RANGE 0.25 TO 1.60, THEABSOLUTE CONCENTRATION OF LACTIC IONS IN SAID BATH EXPRESSED INMOLE/LITER IS WITHIN THE RANGE 0.25 TO 0.60, AND THE INITIAL PH OF SAIDBATH IS WITHIN THE APPROXIMATE RANGE 4.0 TO 5.6.